leap year

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A year in the Gregorian calendar having 366 days, with the extra day, February 29, intercalated to compensate for the quarter-day difference between an ordinary year and the astronomical year.
2. n. An intercalary year in a calendar.

Century Dictionary and Cyclopedia

1. n. A year containing 366 days, or one day more than an ordinary year; a bissextile year. See bissextile. The exact reason of the name is unknown; but it probably arose from the fact that any date in such a year after the added day (February 29th) “leaps over” the day of the week on which it would fall in ordinary years: thus, if March 1st falls on Monday in one year, it will fall on Tuesday in the next if that is an ordinary year of 365 days, but on Wednesday if it is a leap-year.

Wiktionary

1. n. In the Gregorian calendar, a year having 366 days instead of the usual 365, with the extra day added to compensate for the fact that the Earth rotates approximately 365.25 times for each revolution it makes around the Sun.
2. n. In the Jewish calendar or other lunisolar calendars, a year having 13 months instead of 12, with the extra month added because 19 solar years is approximately 19*12+7 lunar months.

GNU Webster's 1913

1. n. Bissextile; a year containing 366 days; every fourth year which leaps over a day more than a common year, giving to February twenty-nine days. See bissextile.
2. n. See Leap year, in the Vocabulary.

WordNet 3.0

1. n. in the Gregorian calendar: any year divisible by 4 except centenary years divisible by 400